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Thread: lens spacing, front/rear using shims

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    Jac@stafford.net's Avatar
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    lens spacing, front/rear using shims

    lens spacing, front/rear using shims

    If we were to add a shim, perhaps .5mm behind a shutter, before the rear lens cluster, therefore increasing cell spacing, how does it effect the focus for accuracy, sharpness? I know there are enormous variables, but I'm just looking for generalities. Hopeless question?

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    Re: lens spacing, front/rear using shims

    Why don't you ask some lenses?

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    Jac@stafford.net's Avatar
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    Re: lens spacing, front/rear using shims

    Quote Originally Posted by Dan Fromm View Post
    Why don't you ask some lenses?
    I don't speak German.

    Seriously, I would need the shims first. I will look around.

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    Re: lens spacing, front/rear using shims

    Quote Originally Posted by Jac@stafford.net View Post
    lens spacing, front/rear using shims
    If we were to add a shim, perhaps .5mm behind a shutter, before the rear lens cluster, therefore increasing cell spacing, how does it effect the focus for accuracy, sharpness? I know there are enormous variables, but I'm just looking for generalities. Hopeless question?

    0.5 mm is a pretty small inter cell distance modification for a LF lens, still it would be noticed in an optical lab, I guess.

    This may change ultimate performace measured at ultimate spatial frequence the lens can resolve, anyway perhaps you are not to notice it much those 0.5mm, and you may not notice it at all when diffraction (lens stopped f/32+) is high.

    You can expect a change in sharpness that is a fraction of the one a process less has when used for distant subjects instead for near subjects, so not much IMHO.


    The lens spacing has to be optimal distance for best correction of aberrations for the intended lens usage. When manipulating inter cells distance, one can optimize sharpness for a near or distant subjects, or for center vs corner performance.

    In non convertible lenses (like old Symmars), my understanding is that front cell is not completely aberrations corrected, and rays get "dispersed" to one radial direction, the rear cell also is also not completely corrected, but error is in the contrary direction than with front cell, so it compensates it. If inter cell spacing is longer or shorter then the error provocated by front cell is larger or smaller, so the compensation result can be brought to a minimum by adjusting inter-cell distance.

    If it was a process lens optimized in the factory for near subjects then by increasing inter cell distance you were to going in the direction to optimize it for distant subject, if I'm not wrong some ULF shooters unscreew a bit front cell of some process lenses to be it better for distant subjects. If you increase the inter-cell distance of a pictorial lens that is yet optimized for distant subjects then you will worsen general performance.

    Also center vs corner sharpness balance may change, in theory with the shim you may slightly favor corners and slightly penalizing center sharpness, but I'm not sure, it can be the counter.

    But you can judge on you own. Just unscreew the front cell the number of tours that give 0.5mm, and use a resolving power target to see ultimate performance, you can use an eyepiece (x10) from a microscope in the camera back (without the groung glass) to see it "in the air".

    I made personal experiments with my lenses in this way... I was provocating aberrations to have a soft image for portrait, then I also was measuring lp/mm with an eyepiece.

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    Re: lens spacing, front/rear using shims

    Quote Originally Posted by Jac@stafford.net View Post
    I don't speak German.

    Seriously, I would need the shims first. I will look around.
    Just back the cells out.

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    Jac@stafford.net's Avatar
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    Re: lens spacing, front/rear using shims

    Quote Originally Posted by Dan Fromm View Post
    Just back the cells out.
    Oh sure, Dan - the reasonable and scientific method! Yer gold, Dan. Seriously.
    Thank you.
    .

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    lens spacing, front/rear using shims

    You'll see aberrations most strongly influenced by stop position increase first, so the first thing you may notice is a degradation in off-axis image quality.

    The degradation can be compensated to some extent by stopping down the lens (not distortion tho), but that will be less effective the farther the cells are separated. Sooner or later, you'll lose the ability to sharply focus even at center (spherical aberration and/or sphero-chromatism).

    The sensitivity all depends on the specific design -- double gauss and cookes are more sensitive to this deviation from nominal separation distance than anastigmats, for example.
    Newly made large format dry plates available! Look:
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    Tin Can's Avatar
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    Re: lens spacing, front/rear using shims

    Good thing you showed up Nudda. I was trying to find an answer in a 1972 SPSE Handbook. It's there and I glazed over your answer.

    A little off topic, but I found a chart of Lens Mounting Distances in a 1951 Photo Tech book.

    I look at the 35 mm FP numbers and wonder why those distances were picked. F-Mount is the closest to a simple number. 46.5 mm. Leica Bayonet is 27.95 mm and yes I know why those are so diiferent. But why is Pentax 45.46 mm? Expiring minds wonder.

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    Nodda Duma's Avatar
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    lens spacing, front/rear using shims

    There's no direct technical reason for differences in flange mounting distances.

    I always figured it mostly boiled down to ensuring you'd buy the lenses made by that specific camera company...i.e. a business decision drives the differences in flange distance.

    That or it was defined by what they achieve for the minimum thickness of whatever their first camera body was that used that lens mount.

    The Leica bayonet number converts to 1.1" (so the random metric values may convert to nice round numbers in inches).
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    Tin Can's Avatar
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    Re: lens spacing, front/rear using shims

    Even if they all were the same they would not interchange. Iím disappointed in the randomness.

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